1. Field of the Invention
The present invention relates to an alloy sheet for shadow mask having high press-formability,
2. Description of the Related Arts
Recent up-grading trend of color television toward high definition TV has employed Fe--Ni alloy containing 34 to 38 wt. % Ni as the alloy for making a shadow mask to suppress color-phase shift. Compared with low carbon steel which has long been used as a shadow mask material, conventional Fe--Ni alloy has a considerably lower thermal expansion coefficient. Accordingly, a shadow mask made of conventional Fe--Ni alloy raises no problem of a color-phase shift coming from the thermal expansion of shadow mask even when an electron beam heats the shadow mask,
The common practice of making the alloy sheet for a shadow mask includes the following steps. An alloy ingot is prepared by a continuous casting process or ingot-making process. The alloy ingot is subjected to slabbing, hot-rolling, cold-rolling, and annealing to form an alloy sheet.
The alloy sheet for the shadow mask is then processed usually in the following steps to form a shadow mask. (1) The alloy sheet is photo-etched to form passage-holes for the electron beam on the alloy sheet for the shadow mask. The alloy sheet for the shadow mask perforated by etching is hereinafter referred to as "flat mask". (2) The flat mask is subjected to annealing. (3) The annealed flat mask is pressed into a curved shape of a cathode ray tube. (4) The press-formed flat mask is assembled to a shadow mask which is then subjected to a blackening treatment.
Since the shadow mask material of conventional Fe--Ni alloy prepared by cold-rolling, re-crystallization annealing, and finish-rolling has a higher strength than a conventional low carbon steel shadow mask material, it is softened by softening-annealing (annealing before pressing) at a temperature of 800.degree. C. or higher temperature for securing the good press-formability after perforation by etching. The softening at a high temperature of 800.degree. C. is, however, not favorable from the viewpoint of work efficiency and also of economy. Accordingly, the industry waits for the development of materials which provide a strength as low as the material having been softened at the temperature of 800.degree. C. or higher even if they are subjected to softening at a low temperature.
Improvement of press-fore, ability of an INVAR alloy for a shadow mask was disclosed in the Japanese Unexamined Patent Publication No. 3-267320. This prior art provides a technology to reduce strength under a low temperature softening annealing at below 800.degree. C., where an alloy is treated by cold-rolling, recrystallization annealing, and finish cold-rolling at the reduction ratio of 5 to 20 wt %. The temperature of softening is below 800.degree. C. The prior art produces a sheet having sufficiently low strength to give good press-formability with the 0.2 wt % proof stress of 9.5 kgf/mm.sup.2 (less than 10 kgf/mm.sup.2) at 200.degree. C. by the softening annealing at the temperature of less than 800.degree. C.
However, the technology disclosed in the Japanese Unexamined Patent Publication No. 3-267320 only focuses on the average grain size and strength, and the disclosed process induces a considerable increase of degree of {100} plane and generates a mixed grain structure. As a result, the shadow masks prepared by the prior art were found to gall the dies during press-forming and easily generate cracks at the edge of shadow masks. In addition, the material prepared by the prior art gave large plane anisotropy to induce a blurred periphery of pierced holes of the shadow mask after press-forming, which raised quality problems.